4B5B code
The 4B5B code is a term from telecommunications that describes a line code that uniquely reversibly maps four useful data bits to five code bits. This code is used for Fast Ethernet 100BASE-TX in combination with MLT-3 line coding for copper cables and NRZI line coding for the Fiber Distributed Data Interface (FDDI). Further areas of application are the Multi Channel Audio Digital Interface (MADI) according to the AES-10 standard.
By inserting another bit, the coded bit rate increases by 25% compared to the useful data bit rate.
4B5B table
| designation | 4B | 5B | function |
|---|---|---|---|
| 0 | 0000 | 11110 | Hex data 0 |
| 1 | 0001 | 01001 | Hex data 1 |
| 2 | 0010 | 10100 | Hex data 2 |
| 3 | 0011 | 10101 | Hex data 3 |
| 4th | 0100 | 01010 | Hex data 4 |
| 5 | 0101 | 01011 | Hex data 5 |
| 6th | 0110 | 01110 | Hex data 6 |
| 7th | 0111 | 01111 | Hex data 7 |
| 8th | 1000 | 10010 | Hex data 8 |
| 9 | 1001 | 10011 | Hex data 9 |
| A. | 1010 | 10110 | Hex data A |
| B. | 1011 | 10111 | Hex data B |
| C. | 1100 | 11010 | Hex data C |
| D. | 1101 | 11011 | Hex data D |
| E. | 1110 | 11100 | Hex data E |
| F. | 1111 | 11101 | Hex data F |
| Q | - | 00000 | Quiet (loss of signal) |
| I. | - | 11111 | Idle (pause) |
| J | - | 11000 | Start # 1 |
| K | - | 10001 | Start # 2 |
| T | - | 01101 | End |
| R. | - | 00111 | Reset |
| S. | - | 11001 | set |
| H | - | 00100 | Stop |
Data symbols are designated with 0..9 and A..F, followed by control codes.
When mapping according to this table, long "0" or "1" sequences, which could make clock recovery more difficult, are avoided . For this purpose, the data words (0..9 and A..F) in the conversion table must not contain more than one leading "0" and no more than 2 concluding "0"; this rule is determined by a combination for the control codes ensured by two control words. Thus the 4B5B code is equal to a (0.3) RLL code .
The control codes are summarized in pairs according to the following table. With Fast Ethernet according to 100BASE-TX, for example, a data frame begins with a JK pair. With FDDI, the data frame ends with a TT pair, with 100BASE-TX with a TR pair.
| designation | 5B symbols | use |
|---|---|---|
| JK | 11000 10001 | Sync, start delimiter |
| II | 11111 11111 | Not used |
| TT | 01101 01101 | FDDI end delimiter |
| TS | 01101 11001 | Not used |
| IH | 11111 00100 | SAL |
| TR | 01101 00111 | 100BASE-TX end delimiter |
| SR | 11001 00111 | Not used |
| SS | 11001 11001 | Not used |
| HH | 00100 00100 | HDLC0 |
| HI | 00100 11111 | HDLC1 |
| HQ | 00100 00000 | HDLC2 |
| RR | 00111 00111 | HDLC3 |
| RS | 00111 11001 | HDLC4 |
| QH | 00000 00100 | HDLC5 |
| QI | 00000 11111 | HDLC6 |
| 00000 00000 | HDLC7 |
literature
- Sidnie Feit: Local Area High Speed Networks . Macmillan Technical Publishing, 2000, ISBN 978-1-57870-113-1 .